Riding power equipment such as a riding mower and a riding grass cutter typically travels a straight course from one end of an area to be worked to another, and covers the entire area by laterally shifting the straight course every time the power equipment reaches either end of the area to be worked. If the power equipment swerves from the straight course, an area that is not worked is created. This can be avoided by reducing the amount of the lateral shift and increasing an overlapping area between adjoining courses. However, this reduces the work efficiency because a larger number of passes are required to cover the entire area. It is therefore important to prevent the riding power equipment from swerving or to ensure a favorable straight traveling performance of the riding power equipment in view of improving work efficiency.
JP2013-1229A discloses an arrangement for allowing a vehicle to continue to travel straight ahead without requiring a steering input by a vehicle operator. When a vehicle traverses a slope an angle to the slope, the vehicle tends to turn in a down slope direction even when the steering wheel is kept in the neutral position owing to the force acting on the wheels in the down slope direction. According to this prior art, the traction torque of the driven wheels are adjusted in dependence on the sloping angle of the vehicle so that the vehicle may be steered as intended by the vehicle operator.
A passenger vehicle is often equipped with an independent suspension system in view of maximizing riding comfort. However, in the case of riding power equipment such as a riding mower, a rigid axle suspension system is most commonly used because of high robustness and low cost. If the axle is rigidly attached to the vehicle body, one of the wheels may be lifted from the ground when traveling on an irregular ground surface. To overcome this problem, the axle of the rear wheels or the front wheels may be pivotally supported by the vehicle body at a central point thereof This system is referred to as “center pivot system” in the following disclosure.
According to the center pivot system, when one of the wheels rides over a bump or drops into a dip, the center pivot axle tilts in a corresponding manner. Therefore, the camber angles of the wheels are changed so that the vehicle is steered in the corresponding direction. More specifically, in the case where the front axle is supported by the vehicle body via a center pivot while the rear axle is rigidly supported by the vehicle body, if the left front wheel rides over a bump, the left end of the front axle is lifted with the result that a positive camber is produced in the right front wheel (the right front wheel tilts rightward). This causes the vehicle to be steered to the right. Therefore, the vehicle operator is required to steer to the left in order to keep the vehicle to travel straight ahead. This requires a constant attention of the operator, and impairs the comfort of the operator.